A common type of construction for electrical wires or cables designed for medium-to-high voltage applications, for example about 15 to 69 KV, as well as other classes of electrical service, comprises combinations of one or more insulating layers and semiconductive layers. In a typical cable structure, for instance, the metallic conductor may be provided with an organic polymeric insulation such as a crosslinked polymer comprising ethylene, and an overlying body of semiconducting material comprising an organic polymeric material which has been rendered electroconductive by the inclusion therein of electrical conductivity imparting agents or fillers such as carbon black. Although these cable constructions may vary in certain elements, and often include intermediate components disposed between the metallic conductor and the primary body of dielectric insulation, such as a layer of separating tape and/or inner layer of semiconductive material, or the overall cable assembly is enclosed within a covering sheath, all such cable constructions conventionally include therein at least a body of primary dielectric insulation surrounding the conductor and an overlying body of semiconducting material in physical contact with the insulation. However, this arrangement of a layer of insulation with a superimposed layer of semiconductive material thereover incurs certain handicaps.
For example, to prevent the occurrence of ionization or corona formation resulting from internal voids or pockets within the cable construction and consequent ultimate breakdown of the insulation, it is necessary to eliminate the presence or possible occurrence of any free space or voids within or resulting from the interface between the adjoining surfaces of the body of the insulation and the body of semiconducting material. U.S. Pat. Nos. 3,541,228 and 3,677,849 deal with this problem of intermediate void spaces at the interface of the insulation and semiconductive material by applying a heat treatment to the assembled product to induce a shrinkage of the semiconductive materials tightly about the insulation. U.S. Pat. No. 3,259,688 proposes a different solution to this problem comprising a distinctive construction and an irradiation treatment.
Further, the insulation layer and overlying semiconductive layer for electrical cable can be formed concurrently about the wire or metal conductor by means of a continuous simultaneous extrusion process with one extruder, such as shown in U.S. Pat. No. 3,646,248, or these layers can be formed in sequence employing tandem extruders such as shown in U.S. Pat. No. 3,569,610, and both layers are thereafter cured at the same time in a single operation and unit to minimize manufacturing steps and apparatus. However, the simultaneous curing of both layers together, or even the curing of only one layer along while it is in a contiguous arrangement with the other layer, can result in the apparent formation of crosslinking bonds bridging across the interface between the adjoining surfaces of each phase as noted in U.S. Pat. Nos. 3,569,610 and 3,792,192. This occurrence of such crosslinking bonds bridging the interface between the surfaces of the phases can render their subsequent separation very difficult, such as during removal of a portion of the body of semiconductive material from about the insulation by stripping for the purpose of making splices or terminal connections.
The separation of these layers often requires the application of great force, and, upon being stripped or peeled off, the semiconductive material often is prone to leave a substantial residue of its mass firmly adhering to the other surface or the insulation. As is known in the art, it is necessary when splicing and treating cable ends that the semiconductive material be cleanly stripped or completely removed from the terminal section of the cable end without any damage or material loss to the underlying surface of the insulation, and consequently the separation of these phases can require an appreciable amount of added labor time and costs when the semiconductive material is difficult to remove by stripping and/or a residue thereof is retained tenaciously adhering to the surface of the insulation. A solution to the difficulties of this aspect of such cable constructions is the subject of U.S. Pat. No. 3,684,821.
Other recent U.S. Pats. addressed to the foregoing problem comprise the following: Nos. 3,643,004 relates to a cable construction wherein the semiconducting layer is adhering but unbonded to the insulating layer; No. 3,787,255 which teaches attaching sulfonate groups to the surface of the polyolefin insulation to deter migration of the curing agent from the semiconductive layer across the interface to the insulation and as a result thereof their tenacious interlocking; and 3,793,476 which proposes a semiconductive composition comprising a novel blend of ethylene-propylene rubber and chlorine-containing polymers which forms a controlled bonding upon curing with the underlying insulation of ethylene-containing polymer.